The present invention relates to a hydraulically operated directional control valve for support mechanisms in underground mining, with a valve piston guided sliding in a boring of a valve seat carrier with a valve cone to abut in a sealing manner a sealing ring arranged in the valve seat carrier and with a control piston guide for the actuation of the valve piston accommodated so as to slide in a control piston, with a high pressure connection, a load connection, a return flow connection and a control pressure connection for hydraulic fluid, whereby on actuation of the valve piston by the control piston the return flow connection can be closed by a closing element of the control piston and following the lifting of the valve cone from the sealing ring a fluid connection can be set up from the high pressure connection to the load connection.
Pressure controlled direction control valves of this construction find application in underground mining in advancing support frames, to extend and retract their hydraulic cylinders. The high hydraulic pressures and the frequently repeated operating processes in ongoing support operations lead to high pressure change loadings in directional control valves, which have the consequence of a high degree of wear especially on the sealing surfaces of the valves.
A 3/2 directional control valve of the construction described in the opening paragraph is proposed in DE 197 08 741 A1, which operates with positive overthrust. This means that the feed to the high pressure connection of the valve is only opened when the return flow connection is fully closed. On operating the previously proposed valve, pressure spikes of up to 1000 bar can arise, which cause a very hard switching and can lead to the cartridge coupling of the valve being loosened or damaged and after only a comparatively short operating time damage occurs to the static seals of the valve cartridge. The previously proposed valve requires a high control pressure of some 230 bar, whereby also the wear is undesirably high on the control piston side.
It is the aim of the present invention to produce a directional control valve of the construction described in the opening paragraph in which the opening and closing proceeds without hard switching processes and thus with substantially reduced wear, without the security of switching being negatively influenced thereby.
Accordingly the present invention is directed to a hydraulically operated directional control valve as set out in the opening paragraph of the present specification, in which the valve piston is guided without sealing between the high pressure connection and the sealing ring in the valve seat carrier and in which the closing element of the control piston closes the return flow connection, before the valve piston releases the high pressure connection for the establishment of the fluid connection with the load connection.
The seal free guidance of the valve piston in the valve seat carrier has the effect that high pressure fluid, which remains continually at high pressure can flow through the non-sealed region between the valve piston and the valve seat carrier immediately after the lifting of the valve piston from the sealing ring, so that in the switching process for opening the valve, owing to the overflow of pressure fluid during the switching process an at least partial pressure equalisation occurs, which has the consequence that the control pressure with which the control piston presses on the valve piston in the sense of an opening movement can be significantly lower than was the case in the previously proposed valves. As a consequence of the overflow of high pressure fluid, beginning already with the lifting of the valve piston from the sealing ring it is possible to close the return flow connection by means of the control piston before the valve piston releases the high pressure connection for the establishment of the fluid connection with the load connection to its full cross section, without pressure peaks during the opening process. This comes about because the high pressure fluid does not stream abruptly into the inside of the valve on the closing of the return flow connection and the further opening movement of the valve piston, but high pressure fluid streams already via the gap between the valve piston and the valve seat carrier and the pressure at the load connection continuously increases with increasing closure of the return flow borings. Although the valve according to the present invention thus appears according to its kinematics as a valve with positive overlap, it belongs in fact according to its method of operation to the valves with negative overlapping, i.e. in closing the return flow the infeed is simultaneously gently opened, so that pressure shocks do not arise or only in much reduced measure and the lifetime of the valve is substantially increased.
An especially simple constructional configuration results if the control piston and the valve piston are joined together. This can be achieved for instance if the control piston is screwed onto the valve piston by means of a threaded connection. It is also possible in this to configure the threaded connection as a separation adjusting mechanism, with which the relative axial positions of the control piston and the valve piston can be changed. By this the point in time at which the high pressure connection is released after the closing of the return flow lines by the valve piston can be varied. This can for instance be effected by the exchangeable arrangement of distance washers between the valve piston and the control piston on the thread root of the threaded connection.
Preferably the control piston preferably has a control piston body as a closing element closing the return flow connection when activated by control fluid under pressure and a control piston shoulder, offset from this, guided in a sealed manner in a control piston guide whereby the control piston body is guided without being sealed in a control piston guide. It is also expedient if the control piston guide is provided with at least one relief hole in the region of the control piston body which is connected to the return flow connection via a relief channel. Both measures ensure an especially light, shock-free operation of the control piston in both the opening and closing processes.
The control piston guide has preferably a diameter which corresponds at least approximately to the diameter of the valve seat carrier, whilst the diameter of the control piston body corresponds at least approximately to the effective opening diameter of the valve cone. By this configuration the directional control valve experiences a pressure equalisation so that for the complete opening of the valve piston essentially the pressure force of its valve closing spring alone has to be overcome and the pressure force of the hydraulic fluid can lift off from the effective surfaces of the control and valve pistons, which are essentially the same size but positioned opposite and with the force balance can remain to the greatest degree without relevance in the switching of the valve. The pressure balance can, of course, also be effected by other suitable means.
Preferably the valve piston is pre-tensioned against the sealing ring by the effect of a closing spring which is arranged in the valve seat carrier. This provides an especially space saving configuration.
The sealing ring can be a plastics material sealing ring in a previously proposed manner. It can expediently be sealed to the valve seat carrier by an interposed O-ring or similar, which effectively prevents the build up of pressure behind the plastics material sealing ring in the closed position of the valve. Preferably the directional control valve is also provided with a control piston sealing ring for the control piston in its open controlled position, which effectively seals the control piston against the inner valve chamber and prevents high pressure fluid flowing by the control piston to the return flow borings in the open position of the valve.
The sealing ring for the valve piston is preferably held in its position in the valve seat carrier by a retaining ring. An especially advantageous configuration results if the control piston sealing ring is configured as an assembly with the retaining ring, where the retaining ring or the control piston sealing ring can comprise a steel sealing ring with a conical sealing surface matched to the front end of the control piston.
The valve piston is preferably axially secured in the valve seat carrier with a spring ring and the control piston is provided with a connecting thread or similar for a disassembly tool. The directional control valve can then be removed as a valve cartridge from its valve housing in the assembly unit where the disassembly tool is screwed onto the control piston or fastened in another suitable manner and then the control piston is pulled out using the disassembly tool, whereby the spring ring lies against the valve seat carrier and pulls this also out of the valve housing.